Final Fantasy X Walkthrough
Gagazet - The Trials of Gagazet

The Trials are in the Mountain Cave and involve only Rikku, Tidus and Wakka. To start with, from the save sphere you have to go left. Swim straight - if you go right at one point you can see 2 chests but you can't get to them. You'll hit some stairs. Go up. You'll hit a light within a spinning ball that Wakka has to hit. This is the First Trial of Gagazet.

This is pretty easy. The thing spins with three cycles - partially shielded, a bit shielded, and wide open. Just watch it spin and wait for the wide open cycle to come around and hit it. You can take as long as you want. A chest appears and you get a lvl 1 key sphere.

OK, return to save spot. Now the north path is open. Go up and left. Follow this to the Second Trial of Gagazet. There are 3 holes that you have to have them swim through. Of course Rikku is small and has to go through the smallest one, and Wakka is big and has to go through the biggest one. Look at the size of each hole and tell each person to go into the appropriate hole. If you fail you can just try again. When you do, go back and save. Now you can go right to the 2 chests and get a recovery ring and a return sphere.

If you head down the first path (towards Trial 1) the area to the right is now open and in a chest you get a Pep Talk. OK, save again at the save sphere.

Now head north again, head left (as if you were going towards Trial 2) but stay to the left of the passageway and go up the narrow path. You learn that Yunalesca is waiting for you in the destroyed city of Zanarkand, and Auron sort of taunts Yuna. There's a save globe here. SAVE!!! You're about to hit a boss.